Solids invasion and formation impairment can be minimized by adding bridging materials to drilling muds. Measurement of size distributions of solids in the muds along with calculations of rockpore sizes can be used to design competent muds. Test results of a laboratory mud agree well with published results.

Introduction

Although invasion and formation damage occur with all muds during drilling, it is possible to minimize solids invasion and formation impairment by adding bridging material to the muds. This bridging material is chosen by matching its size to the formation-rock pore sizes. Techniques (particularly the Coulter counter method) are now available for measuring the size distributions of solids in the muds. This measurement, coupled with calculation of rock pore sizes from permeability or capillary pressure data, forms the basis of the matching procedures reported here for the design of competent muds that is, muds that impair to depths normally penetrated by perforations.Two rules are used for selecting the size and concentration of bridging materials.1. The median particle size of the bridging additive should be equal to or slightly greater than one-third the median pore size of the formation.2. The concentration of the bridging size solids must be at least 5 percent by volume of the solids in the final mud mix.Laboratory tests with competent muds show that impairment caused by invasion of mud particles occurred to a depth of less than 1 in., a depth normally penetrated by perforations. Levels of impairment observed do not significantly affect injectivity or productivity of the systems studied. A laboratory mud was tested on 4- to 6-darcy unconsolidated sand in a radial-flow apparatus. Also, a Wasson San Andres field mud was tested in linear flow tests on 5- and 50-md dolomite cores taken from the Wasson field. Where comparisons are possible, these results agree with those for similar tests reported in the literature.

Results of Studies

Results from investigations of the effects of drilling-mud particle invasion on rock permeability can be summarized as follows.1. Invasion and formation damage occur with all muds.2. The depth of invasion and level of impairment can be controlled, to a certain degree, by designing the mud to include bridging material.3. The effectiveness of the bridging material in reducing invasion is a function of the concentration and particle size of the material and of the pore sizes of the formation rock.4. Damage is most likely to occur in higher permeability formations; most muds contain sufficient quantities of particles, including cuttings, in the size range required to bridge lower-permeability rocks.5. Where invasion occurs, backflushing does not remove the impairment.These results are based primarily on tests performed on linear, consolidated cores using fluids that in many instances were not designed to minimize invasion. Although depths of invasion as great as 12 in. have been reported, impairment with competent muds has been found to occur within several inches of the wellbore.

JPT

P. 586^

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